1. Field of the Invention
The present invention relates to a method of making an electrode for an electrochemical capacitor such as an electric double layer capacitor, and a method of making an electrochemical capacitor.
2. Related Background Art
Electrochemical capacitors such as electric double layer capacitors can easily achieve a smaller size and a lighter weight, and thus are expected to become power supplies or backup power supplies for portable devices (small-size electronic devices), auxiliary power supplies for electric cars and hybrid cars, etc., for example. Therefore, various studies have been under way in order to further improve their performances (capacities, voltage resistance characteristics, cycle characteristics, and stability during high-temperature storage). For further improving performances of electrochemical capacitors, electrodes in the electrochemical capacitors are also desired to improve their performances.
As a technique for making an electrochemical capacitor electrode and electrochemical capacitor in order to further improve characteristics of the electrode and the electrochemical capacitor equipped therewith, one subjecting a carbon material (after activation, such as activated carbon), which becomes an electrode active material, to thermal plasma processing and purifying the surface of the carbon material has been proposed in order to lower irreversible capacity in particular (see, for example, Japanese Patent Application Laid-Open Nos. HEI 10-92432 and 2000-223121).
Though no detailed mechanisms have been elucidated, electrochemical characteristics of electrochemical capacitors such as reversible capacity, voltage resistance characteristics, cycle characteristics, and stability during high-temperature storage are known to be greatly affected by the degree of crystallization (degree of graphitization), surface form, internal structure, surface chemical composition, absorbed moisture amount, and the like of particles made of a carbon material to become an electrode active material in general. Also, the amount of oxygen-containing functional groups such as carboxyl and carbonyl groups on particle surfaces of the carbon material is known to have a strong influence on the electrochemical characteristics.
The above-mentioned technique aims at purifying the surface of the carbon material to become an electrode active material and allowing the surface to attain physical and chemical states suitable for yielding sufficient electrochemical characteristics by thermal plasma processing.